1. Field of the Invention
The present invention relates generally to apparatus employing ultrasound to examine an object and, more particularly, to an ultrasound probe and a drive system.
2. Description of the Prior Art
Physicians use apparatus commonly referred to as "ultrasonic scanning systems" to aid them during their examinations of patients. With an ultrasonic scanning system, a physician can obtain an image of a portion of a patient that is of interest. Further, ultrasonic scanning systems are used in a variety of nonmedical applications to obtain images of objects or portions of objects.
An important component of an ultrasonic scanning system is a piece of apparatus commonly referred to as an "ultrasound transducer probe", an "ultrasound probe" or, simply, a "probe". The ultrasound probe is used to direct into the patient a timed series of pulses of ultrasound energy ("incident beam") and to receive and convert to electrical signals the series of ultrasound echoes reflected from acoustical interfaces located within the patient ("echo beam"). The nature of the ultrasound echoes and the nature of the electrical signals derived from those echoes are indicative of the nature of the acoustical interfaces which reflected them. Accordingly, proper processing of electrical signals derived from an echo beam yields a display that shows a point of each acoustical interface encountered by the incident beam. Similarly, a number of incident beams directed through a section of the patient produces a corresponding number of returning echo beams which can be processed to create an image of that section of the patient.
One type of ultrasound probe (referred to hereinafter as a "moving crystal probe") employs a single moving ultrasound crystal (the element that creates the pulses of ultrasound energy), several moving crystals, or a single moving crystal having several electrically distinct beam-producing regions (also known as an annular array crystal). Such a probe must have apparatus for moving the crystal to cause it to direct a series of incident beams through a section of the patient. Most ultrasound probes designed to date use a conventional electric motor that oscillates or rotates a crystal to cause the probe to scan a sector of the patient. One probe, which is disclosed in U.S. Pat. No. 4,092,867, has a crystal mounted directly to a magnet that is mounted for rotation between two legs of an electromagnet. The direction of the current applied to the electromagnet is periodically reversed to cause the magnet and crystal to oscillate and permit the crystal to scan a sector of a patient. Another type of ultrasound probe (referred to hereinafter as a "moving mirror probe") employs an acoustical mirror and one or more stationary crystals. The acoustical mirror is moved to cause the incident beams generated by the stationary crystal to be scattered through a sector of the patient under examination. Moving mirror probes are preferrable to moving crystal probes under certain well-known conditions for example, when the probe is designed to employ an annular array crystal. Generally, a moving mirror probe eliminates the problems associated with commutating to a moving crystal--and, in particular, an annular array crystal--electrical energy needed by the crystal to generate ultrasound pulses and to commutate from the crystal electrical signals created from echoes received by the crystal. Again, conventional electric motors are usually used to move the acoustical mirror.
Two problems are associated directly with the use of conventional electric motors in ultrasound probes. First, the motion of the motor must be transferred to the mirror or crystal by some sort of mechanical linkage. Such a transfer of motion does not permit the precise position control of the mirror or crystal that is required to produce high quality ultrasound images. Also, the linkage causes mechanical vibrations that cause the probe to vibrate and further introduces errors into the information transferred by the probe to the video equipment of the scanning system--both of which adversely affect image quality by misregistering the locations of image features.
Control of mirror or crystal motion is accomplished generally--if at all--with electrical control of motor motion. Conventional electric motors and control schemes, however, have not provided a completely satisfactory solution to the problem of precise motion control of acoustical mirrors and crystals. Many stepping motors are capable of stopping only at a number of angular increments equal to the number of wound poles or equal to a small multiple of the number of poles. Commonly, digitally controlled stepping motors are capable of stopping at 200 positions per revolution, or at 360/200 degrees per increment. However, extremely precise motion control and stopping ability--which are required of ultrasound probe motors to enable probes to produce high quality images--would require an impractically large number of coils. Further, a stepping motor having a large number of poles would have limited angular velocity due to the limitations imposed by the required switching frequency. Moreover, the rotor of a stepping motor cannot be made to accelerate and decelerate frequently at approximately 2,000 radians/second.sup.2 (the level of acceleration and deceleration that an ultrasound probe motor must achieve) without risking damaging the motor because the stepping motor must carry a high inertia permanent magnet energy field.
DC motors having small low inertia printed circuit rotors have been used in the types of control systems employing an optical shaft encoder. However, such motors have two sided field magnet assemblies that are physically large and, accordingly, do not provide the performance that an ultrasound probe motor must provide to permit the probe to produce a high quality image. Further, such motors must be sealed from such harmful working environments as the ultrasound transmissible liquid of an ultrasound probe to avoid interaction of the commutation contacts of the motor with the environment. For example, any conventional DC motor in a servo control loop must be isolated from corrosive or combustible liquids or gases to avoid combustion or damage to the commutating devices within the motor. Many conventional low inertia DC motors that are used with electronic commutation designs provide indirect access from the rotor to the load due to intervening bearings or overhanging magnet assemblies.
Accordingly, there exists a need for an ultrasound probe having a moving acoustical mirror, the position of which can be controlled precisely at all times. Further, there exists a need for a drive system which can be used to move an ultrasound crystal or acoustical mirror of a probe in a precise manner. In particular, there exists a need for an ultrasound probe drive system which provides rapid positioning and accurate stopping of a crystal or mirror. Ideally, the drive system should be able to stop a rotating mirror or crystal at a position that is less than 0.1 degree from a commanded position and should be able to accelerate at greater than .+-.2,000 radians/second.sup.2. The drive system should permit access to the mirror or crystal and there should be minimum overhang of motor electromagnets, field magnets, or commutation devices. Preferably, the bearing system of the drive system should be lubricated directly by the hostile fluid environment in which the drive system is immersed.